This invention relates to electric heaters and their manufacture and more particularly to tubular heaters of a type usable as engine glow plugs or for other localized heating applications and having a conductive tubular sheath surrounding an electrical heating coil.
It is known in the electric heater art to provide heaters for use as engine glow plugs and the like which have a closed end tubular protective metal sheath surrounding an axially extending heating element. The element is electrically connected with the closed end of the sheath and also connects with an electrode extending from the sheath open end. The remainder of the sheath interior is packed with a suitable electrical insulating material such as magnesium oxide (MgO).
A prior method of manufacturing such heaters has included a step of hot welding the end of the coiled heating element to the closed end of the sheath in order to provide a satisfactory electrical connection between the heating element and the sheath. Even though this welding is carefully done, localized metallurgical changes may result in resistance variations and susceptibility to cracking during a subsequent swaging operation. This may affect the usability of the resulting part. Another problem of such heaters in service is caused by internal oxidation of heating element, possibly due to oxygen generated from the moisture content of the magnesium oxide of from other external sources. Such oxidation may result in excessive localized heating and the eventual breakdown of the heating element.